simulating the evolving milky way: from disk to halo · 2017-10-24 · simulating the evolving...

Simulating the Evolving Milky Way: From Disk to Halo

Sarah R. LoebmanHubble Fellow

Andrew Wetzel, University of California, Davis

Victor P. Debattista, David L. Nidever, Michael R. Hayden, Jon Holtzman,Adam J. Clarke, Rok Roškar, Monica Valluri, Kohei Hattori, Eric Bell, Greg

Stinson, Alyson Brooks, Charlotte Christensen

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Galactic archeology: what can we learn about MW's past from stars?

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Galactic simulations: stars across time & differing perspective

Latte, Wetzel+2016

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OUTLINE: 1) Stellar Disk & 2) Stellar Halo

a) TheoryRadial Migration

b) ObservationsAPOGEE

a) PredictionsOrbits in ΛCDM

b) Future ObsGaia DR2

2) Stellar Halo

1) Stellar Disk

Latte, Wetzel+2016

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Disk Galaxies

- Disk galaxies are ubiquitous Majority of blue galaxies and up to ~85% of red galaxies (e.g. ATLAS3D, Emsellem+2011)

- Found at a range of stellar mass 107~1011.5 M

sun

- Majority contain bars 50–75% in the near IR (Knapen 1999; Eskridge et al. 2000)

- Majority of massive blue galaxies likely contain both bars and spiral structure

- Generally thought to form inside-out (e.g. Fall & Efstathiou 1980)Recently being reconsidered...

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Star Formation & Heating/Movement

Range of ways to heat stars tomake orbits more eccentric 1) interaction bar structure2) bouncing off molecular cloud3) feedback from star formation4) satellite accretion5) dark matter sub halo heating

only one mechanism thatchanges the guiding center with-out signifcantly kinematicallyheating stars: radial migration.

Stars in disk galaxies are traditionally thought to stay close to their guiding center of formation;

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Radial Migration

Transient spirals can rearrange orbital angular momentum without causing kinematic heating.

Sellwood & Binney 2002

Questions for galaxy evolution:- (How) does it affect disk structure? Thick disk? Outer disk?- Can it smooth rotation curve? Change surface density profle?- Role in vertical and radial metallicity profles?

- Has radial migration signifcantly impacted the Milky Way?- Could the Sun have radially migrated?- Modify "galactic habitable zone?"

Reconsider where life could form?

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Physics of Radial Migration

corotation - where the circular orbital frequency of thedisk equals the spiral pattern speed

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Physics of Radial Migration

corotation - where the circular orbital frequency of thedisk equals the spiral pattern speed

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Physics of Radial Migration

corotation - where the circular orbital frequency of thedisk equals the spiral pattern speed

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corotation - where the circular orbital frequency of thedisk equals the spiral pattern speed

Physics of Radial Migration

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corotation - where the circular orbital frequency of thedisk equals the spiral pattern speed

Physics of Radial Migration

Orbital frequency slower than spiral pattern speed => drifts behind

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corotation - where the circular orbital frequency of thedisk equals the spiral pattern speed

Physics of Radial Migration

Decrease in orbital angular momentumimplies decrease in mean orbital radius

R

F

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corotation - where the circular orbital frequency of thedisk equals the spiral pattern speed

Physics of Radial Migration

Orbital frequency faster than spiral pattern speed => drifts ahead

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Movie courtesy of Rok Roškar

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Roškar et al. (2008)

Simulation: large radial movement in short time & remains circular

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Motivation Radial Migration is a very cool dynamical effectWe'd like to know how much its influenced the MW

ProblemPresent day kinematics (orbital eccentricity)doesn't give us an obvious way to find migrators

Solution Look to other (immutable) quantities.

Chemistry (APOGEE)

Radial Migration in the Milky Way

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APOGEE – Largest ever coverage of the Galactic Disk

chemistry & kinematics: high-res, high-SN spectra ~150,000 giants

● C

What can we learn about stars as a function of radius?

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Consider MW's Metallicity Distribution Function (MDF)

MDF is a histogram of [Fe/H]

Image Credit: 2MASS/J. Carpenter, M. Skrutskie, R. Hurt

R

Z

Select N stars in a region of R & z

[Fe/H]

Nbin

MDF

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What is MDF skewness?

MDF skewness is the direction of the tail relative to peak

Image Credit: 2MASS/J. Carpenter, M. Skrutskie, R. Hurt

R

Z

Look at the global shape of the distribution

[Fe/H]

Nbin

MDF

negative positive

SkewedNegative

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What is the shape of MW MDFs if stars don't radially migrate?

Simple chemical evol models suggest MDFs skewed negatively

Image Credit: 2MASS/J. Carpenter, M. Skrutskie, R. Hurt

R

Z

Simple models: constant MDF shape regardless of R & z

Hayden et al. 2015

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Surprising results in mid-plane

MDF skewness changes with radius● Skew-negative in inner Galaxy● Roughly Gaussian at Solar Cylinder● Skew-positive in outer Galaxy

APOGEE: MW's MDF changes skewness

Hayden et al. 2015

R~5 kpc

R~8 kpc

R~14 kpc

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APOGEE: MDF skewness shift decreases out of mid-plane

Hayden et al. 2015

Increasinglysimilar shape

out of midplane

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APOGEE: high-[α/Fe] MDFs the same at all R and z

Hayden et al. 2015

high-[α/Fe] MDF has

similar shapeeverywhere!

unexpected!(well mixed ISM)

high-[α/Fe]stars form atearly epoch

during vigorousstar formation

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Valid theory of disk formation must reproduced APOGEE results

A successful theory of MW disk formation must:

● Shift the MDF skewness in the mid-plane with increased radius

● Decrease shift in the MDF skewness out of the mid-plane

● Create globally similar MDF for high-[α/Fe] population

Standard disk growth (inside-out, no radial mixing) does not work

26Courtesy of Rok Roškar Gasoline (hydro+Nbody), cosmo-motivated

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All stars in the simulation are born on in negatively skewed MDF

Loebman et al. 2016

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End simulation, MDFs at large radius are positively skewed

Loebman et al. 2016

4.5 < Rfinal

< 5.5 kpc11 < R

final < 13 kpc

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Simulation & APOGEE MDFs qualitatively the same!

Loebman et al. 2016

Hayden et al. 2015

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Simulation & APOGEE skewness parameters match

Loebman et al. 2016

No fine-tuning:Skewness

Parameters match!

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Mid-plane: skewness parameter connected to migrated fraction

Loebman et al. 2016The golden science resultfrom this talk!

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In situ & migrated MDF add up to make total MDF

Loebman et al. 2016Total MDF:Superposition of in situ & migrated

Offset peaks

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APOGEE & Simulation MDFs have same out-of-plane trend

Hayden et al. 2015

APOGEESimulation

Loebman et al. 2016

Tow

ard

sim

ilar

sha

pe

& p

eak

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APOGEE & Simulation high-[α/Fe] MDFs match

Loebman et al. 2016

Simulation

Hayden et al. 2015

APOGEE

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Simulation high-[α/Fe] stars form at roughly same time

Loebman et al. 2016

36Loebman et al.2016

Simulation high-[α/Fe] stars form at roughly same place

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APOGEE high metallicity stars too old to have formed locally

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Conclusion: MW most likely influenced by radial migration

Disk Summary

Radial Migration reproduces key APOGEE results:

✔ Shift the MDF skewness in the mid-plane with increased radiusIncreasing fraction of migrated stars at large radii

✔ Decrease shift in the MDF skewness out of the mid-planeFewer and fewer in situ stars

✔ Create globally similar MDF for high-[α/Fe] population

Stars formed in local reservoir & then migrated!

✔ The highest [Fe/H] stars are on average older than mid [Fe/H] starsThese stars formed in the inner disk in the past and migrated

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SDSS footprint

Part 2: Stellar Halo

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V. Belokurov and the Sloan Digital Sky Survey

SDSS Milky Way Field of Streams

MW's stellar halo a diffuse sphere, littered with dwarf galaxy tidal debris

41Jurg Diemand and the Via Lactea Project, http://www.ucolick.org/~diemand/vl/

Motion of stars in MW stellar halo to find DM mass, accretion history?

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β: statistical measure of how stars orbit in the stellar halo

orbits are tangentially biased

β < 0

β = 0 orbits are isotropic

β > 0

orbits are radially biased

bin by r

Gaia

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Cunningham et al. 2016

β all over the place in the MW

Outside 16 kpc, 4D measurements likely biased tangentiallyHattori et al. 2017

Radial

Isotropic

Tangential

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Simulations enable exploration of the build up of the stellar halo & β

Bullock & Johnston 2005

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radial

tangential

isotropic

radial

tangential

isotropic

Accretion only: Strongly radially biased

Hydro sims: Most radially biased

Hydro outlier

Simulations radially biased

Loebman et al. 2017, submitted

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Loebman et al. 2017, submitted

Merger boosts σΦ above σ

r – persists until present day

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S. R. Loebman, AAS Abstract 134.07

Loebman et al. 2017, submitted

Movie of courtesy of A. Brooks

Major merger z~1

Merger boosts σΦ above σ

r – persists until present day

β Gaia accessible – exciting possibility for understanding past!

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Loebman et al. 2017, submitted

What causes short term β dips?

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Loebman et al. 2017, submitted

Stars from the disrupted satellitecause a short-liveddip in β

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Conclusion: β & Gaia holds potential for understanding merger history

Halo Summary

Simulations make strong predictions for expected β:

✔ ΛCDM predicts strongly radial βHalo stars retain memory of their radial infall

✔ Tangential β over a wide range of radii could indicate major merger Proto-disks nearly aligned

✔ Tangential β over a small range of radii could indicate recent accretion

Spectroscopic follow-up to confirm

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Thank you!Questions?